High density integrated circuit (IC) packages that house LSI/VLSI type semiconductor devices are well known. Input/output pins for such IC packages are often arranged in such a dense pattern (sometimes more than two hundred closely spaced contacts) that direct soldering of the IC package to a substrate, such as a printed wiring or circuit board (PCB) creates several significant problems related to inspection and correction of any resulting soldering faults. Land grid array (LGA) connectors are known for interconnecting IC packages to PCB's. LGA's typically do not require soldering procedures during engagement with the PCB. Prior art LGA assemblies are used to interconnect an IC package having a plurality of flat contact pads or solder bumps formed on a bottom surface, to contact pads arranged in a regular pattern on a surface of PCB.
Prior art LGA assemblies are known which include an insulative housing and a plurality of resilient conductive contacts received in passageways formed in the housing.
Resilient conductive contacts typically have exposed portions at the upper and lower surfaces of insulative housing for engaging flat contact pads. When IC package is accurately positioned in overlying aligned engagement with PCB, such that conductive pads engage conductive pads, a normal force is applied to the exposed portions of each resilient conductive contact to electrically and mechanically engage the respective contact pads.
The resilient conductive contacts associated with prior art LGAs have had a variety of shapes. A commonly used form of resilient conductive contact includes two free ends connected by a curved, resilient portion which provides for the storage of elastic energy during engagement with the IC package and PCB. Prior art resilient conductive contacts may be a single metal structure in the form of a spring to provide the required elastic response during service while also serving as a conductive element for electrical connection. Alternatively, contact buttons have been developed in which a connector is wound around, embedded, or otherwise engaged with a dielectric core, which often provides for elastic energy storage during operation with the conductor merely providing an electrical conduction pathway. Typically, a combination of barrier metal and noble metal platings are applied to the surface of the spring for corrosion prevention and for electrical contact enhancement. It is often the case that these platings are not of sufficient thickness for electrical conduction along only the surface of the spring. Examples of such prior art resilient conductive contacts may be found in U.S. Pat. Nos. 2,153,177; 3,317,885; 3,513,434; 3,795,884; 4,029,375; 4,810,213; 4,820,376; 4,838,815; 4,922,376; 5,030,109; 5,061,191; 5,101,553; 5,127,837; 5,215,472; 5,228,861; 5,232,372; 5,308,252; 5,350,308; 5,385,477; 5,403,194; 5,427,535; 5,441,690;5,473,510; 5,495,397; 5,599,193; 5,653,598; 5,791,914; 5,800,184; 5,806,181; 5,810,607; 5,817,986; 5,823,792; 5,833,471; 5,949,029; 6,074,219; and 6,264,476. The foregoing patents are hereby incorporated herein by reference.
A problem in the art exists in that, a conductive contact in the form of a single conductor spring for attaining high compliance, has a high resistance and a high inductance. Further, a contact, in the form of multiple conductors in a bunched wire bundle, or in the form of a conductor structure embedded in a polymer core, is made with lower resistance and inductance than a single conductor spring, but requires a high contact force for deflection, and is unable to attain high compliance. Further, one problem in the art exists in that a good material for the construction of a spring, such as a high strength steel, is not a very good electrical conductor. On the other hand, a good electrical conductor, such as a copper alloy or precious metal, often does not provide adequate spring properties. There is a need for a more resilient conductive contact which incorporates the seemingly opposing requirements of good spring properties, temperature resistance, and high conductivity, but without the need for any integral supporting structure. Therefore, an improved electrical contact for use in an LGA socket or electrical connector is needed which can overcome the drawbacks of conventional electrical contacts.
Thus, it is desirable that a good electrical contact element possesses the following attributes: (a) usable for both a production socket, as well as, test and bum-in sockets, where the latter use requires high durability; (b) a large elastic compliance range and low contact forces; (c) capable of transmitting high frequency signals and high currents; (d) capable of withstanding high operating temperatures; and (e) exhibiting high durability, i.e. >500K repeated deflections.
The prior art has been devoid of at least one of the foregoing attributes necessary for a universally applicable electrical contact.